Red pandas (Ailurus fulgens) are small arboreal carnivoran mammals native to Asia that are not true pandas but are close relatives. Red pandas are unique among carnivores in many senses: they spend much of their time in trees, they eat mostly bamboo, and they practice a polygamous (both males and females have multiple mates) mating system with little male-male competition. This mating system has historically been pointed to as the reason for the monomorphism between the sexes (males and females are the same size), a rare character not seen in many other carnivores. Considering this polygamous mating system is unusual amongst carnivores, the natural question that comes to mind is when and why did this shift occur in ailurids?
Recently PE authors Ethan Fulwood and Steven Wallace investigated the evolution of monomorphism in ailurids by examining two specimens of the fossil red panda Pristinailurus bristoli from the Hemphillian (late Miocene) Gray Fossil Site of northeast Tennessee. The two specimens are mostly complete and appear to show a significant difference in size, despite their similar morphology and close occurrence in time and place. Lead author Ethan Fulwood writes that “one of the skeletons is noticeably larger than the other, but seemed to be younger than the smaller skeleton.” This intriguing observation led the authors to wonder if the apparent difference in size between these two skeletons was really greater than the difference in size between any randomly drawn pairs of Ailurus.
To test the significance of the observed size difference in P. bristoli the authors calculated the pairwise difference in size between inter-sex and intra-sex pairs of the living red pandafrom the skeletal collections of the US National Museum of Natural History and the Field Museum of Natural History. Using the calculated values from the comparison sample of extant red pandas the authors then compared the distribution of these differences to the difference between the two fossil skeletons. Their findings shows that the two fossil individuals of P. bristoli are significantly more dimorphic.
This apparent size dimorphism could mean a multitude of things. The first and most likely scenario is that the size dimorphism is a sexual size differences in Pristinailurus, indicating a more polygynous (males have multiple mates) mating system. This dimorphism could also reflect the presence of multiple species or subspecies. Fulwood also notes that “what appears to be dimorphism in two skeletons could turn out to represent two individuals at endpoints of a continuous size distribution that is simply larger than that exhibited by living red pandas. Continuing work at the Gray Fossil Site will be needed to uncover enough material of P. bristoli to establish a more complete population profile.” The authors sexual dimorphism interpretation has major implications for the evolution of red panda social systems. While extant red pandas partake in a polygamous mating system this behavior must have evolved more recently than the fossil red panda P. bristoli, which appears to have had a more polygynous mating system. It may take a few more fossil ailurids before the evolution of red panda social behavior is fully understood.